This invention relates to a fuel injection valve for internal combustion engines, particularly for diesel engines.
In a conventional fuel injection valve, during the injection stroke there occurs an increase in the volume of the effective fuel space within the injection pipe and the fuel injection valve, which extends between the fuel injection pump and the nozzle holes, due to lifting of the nozzle needle. This volumetric increase causes a temporary drop in the pressure within the pressure chamber, which in turn affects the injection rate waveform. That is, as indicated by the broken line in FIG. 2, a drop occurs in the lifting speed shortly after the start of injection to form a shoulder a in the waveform, and thereafter the rise speed recovers. This curve resembles the leading edge of an injection rate waveform obtained by a throttle nozzle. However, while in the throttle nozzle the flow rate through the nozzle holes can be throttled along a stable curve at the beginning of lifting of the nozzle needle, in the case of a hole nozzle, the length of the shoulder a is not constant since it can easily be affected by changes in the injection pipe pressure, making it difficult to control the leading edge to a desired definite shape, resulting in unstableness of the injection quantity. Also at the termination of fuel injection, there occurs a decrease in the volume of the effective fuel space due to descending of the nozzle needle to its seated position by the force of the nozzle spring against the force of the pressure within the pressure chamber, which causes a temporary rise in the pressure within the pressure chamber. As a consequence, as shown by the broken line in FIG. 2, the descending speed temporarily drops shortly before the termination of injection. This results in unnecessary injection of fuel immediately before the termination of injection, badly affecting the emission characteristics of the engine.
In order to overcome such disadvantages, the present applicant has previously proposed in U.S. Ser. No. 677,879 filed Dec. 4, 1984, now abandoned a fuel injection valve for an internal combustion engine, which comprises a nozzle holder having a fuel inlet port formed therein and connected to an injection pipe extending from a fuel injection pump, a nozzle body supported by the nozzle holder and having at least one nozzle hole and a pressure chamber formed therein at an end thereof remote from the nozzle holder, the pressure chamber being more remote from the injection pipe than the fuel inlet port, fuel passage means formed in the nozzle holder and the nozzle body and extending between the fuel inlet port and the pressure chamber, a nozzle needle mounted within the nozzle body and liftable and returnable to open and close the nozzle hole, respectively, in response to an increase and a decrease in the pressure of fuel supplied into the pressure chamber, nozzle spring means urging the nozzle needle in a direction of closing the nozzle hole, and a central plunger disposed in the nozzle holder for displacement in unison with the nozzle needle through a whole lifting stroke thereof as well as through a whole returning stroke thereof. The central plunger has an end face remote from the nozzle needle disposed to receive pressure within the injection pipe through the fuel inlet port, to thereby impart an urging force to the nozzle needle in the direction of closing the nozzle hole.
In the proposed fuel injection valve, immediately upon the start of the injection, the central plunger lifts in unison with the nozzle needle in permanent contact therewith to move into the fuel inlet port to cancel an increase in the volume of the effective fuel space which would otherwise be caused by lifting of the nozzle needle, thereby obtaining an injection rate waveform with a sharp leading edge as shown by the solid line in FIG. 2. Further, at the termination of the injection, the central plunger immediately moves back in unison with the descending nozzle needle to recede from the fuel inlet port to cancel a decrease in the volume of the effective fuel space which would otherwise be caused by descending of the nozzle needle, thereby obtaining a sharply cut trailing edge of the waveform as shown by the solid line in FIG. 2. Therefore, the proposed fuel injection valve can achieve an ideal injection rate waveform and permits precise adjustment of the waveform so as to obtain an accurate injection quantity.
However, according to the above proposed fuel injection valve which is constructed such that the nozzle needle sharply lifts and sharply descends, the injection rate is high at the start of the injection due to sharp lifting action of the nozzle needle, and accordingly, the throttling period is short. This is disadvantageous in reducing the combustion noise of the engine.